Analog-to-digital conversion is used in a wide variety of electronics applications--from testing and measurement to entertainment electronics and beyond. In modern practice virtually any electronics application that involves real-word (analog) signals such as sounds, light and temperature employs treatment of corresponding signals in digital form for some part of its operation. Thus, for example, music compact discs store and make available for retrieval digital versions of what normally originate as analog voice or instrument signals. Upon retrieval, such stored signals typically return to their original analog form for playback.
Conversion of signals between the analog and digital domains is therefore a well-developed art. In a larger sense, analog-to-digital (A/D) conversion relates to the conversion of a continuous variable to a nearest discrete approximation, which approximation is generally represented as a multi-bit binary number. See, for example, P. Horowitz and W. Hill, The Art of Electronics, Cambridge Univ. Press, Cambridge, 1989, pp. 612-641.
Many prior analog-to-digital (A/D) converters employ voltage sample-and-hold techniques. These prior techniques thus employ a voltage signal sampled at an instant within a longer period, rather than using a signal averaged over the period. In such sampled operation, noise present at the time of sampling can significantly affect the value used in conversion.
Other structures characteristic of prior A/D architectures include subtractors or other comparator circuitry, or they include explicit D/A converters and other interstage circuitry, all of which tend to increase complexity, required device area and, consequently, cost. Moreover, when traditional architectures perform subranging without subtraction and inter-stage DACs they typically suffer from a requirement that the common-mode voltage range of fine conversion(s) must extend over the entire common-mode range of the coarse conversion(s), thereby degrading speed of operation.
Another phenomenon suffered by traditional architectures arises when comparator s, which are ubiquitous in A/D converters, hover indefinitely between their stable `0` or `1` states. Such metastability arises when input and reference voltages of the comparator are very near to each other, thus causing internal positive and negative currents in the comparator to be almost balanced; such near-perfect balance results in very slow operation of the comparator and causes digital decisions to be made on premature analog values.
In my incorporated patent application entitled Spike-Based Hybrid Computation I describe, inter alia, a comprehensive structure, organization and methodology for a novel hybrid state machine that includes both analog and digital structures. Such computational structures convieniently employ spiking neuron circuits modeled generally on neuron functioning in animals. Illustrative circuits described in this incorporated patent application include so-called neuron circuits for accumulating analog current signals over a period of time and the generation of fast-rising spiking signals as output. Moreover, many traditional analog and digital circuits are advantageously avoided in my new computation architecture by using flip-flops, counters and other circuit elements adapted to receive spiking inputs of the type generated by the above-noted neuron circuits. Important advantages arise from the use the spike-based computing architecture, and circuit elements, as described in the incorporated patent applications.
A further aspect of the incorporated patent application entitled Spike-Based Hybrid Computation is the use in some cases of analog-to-digital conversion techniques, e.g., in restoring analog signals to avoid significant degradation during transmission and processing over time. This restoration is convieniently performed as an analog-to-digital conversion followed by a digital-to-analog conversion. Thus, in the context of hybrid computation of the type described in this incorporated patent application, a simple digital-to-analog (D/A) converter is used to convert the A/D-converted value (from binary form) back to a continuous value--thus implementing an analog-to-digital-to-analog (A/D/A) conversion. It will be appreciated that such use of an A/D/A process effectively performs a rounding operation.
Because my spike-based hybrid computing architectures and circuitry achieve important performance and flexibility advantages, and because prior A/D converters are not well adapted for use with spiking neuron circuits and other spike-based circuitry, a need exists for Current-Mode Spike-Based Analog-to-Digital Conversion circuits and methods.